In direct injection engines, the engine control system operates the engine in both a stratified mode and a homogeneous mode. In the stratified mode, which is typically used during low or mid load operation, the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. In the homogenous mode, which is typically used in medium or high load operation, a mixture with a relatively constant air/fuel ratio is present throughout the cylinder volume.
Fuel vapor recovery systems are employed on motor vehicles and need to be combined with direct injection engines to reduce atmospheric emissions of hydrocarbons by storing the hydrocarbons in a canister. The canister, which is coupled to the fuel tank, uses activated charcoal for absorbing the hydrocarbons. The canister is periodically purged by passing ambient air, which desorbs the hydrocarbons, through the charcoal. The resulting air and hydrocarbon mixture subsidizes the normal mixture of air, from the intake system, and fuel, from the fuel delivery system, inducted into the engine via the engine port. The canister is then able to again store hydrocarbons allowing the process to repeat.
In direct injection engines, purging is typically disabled when operating in the stratified mode. However, the fuel vapor recovery process must be executed at regular intervals to assure that the canister does not become saturated. Therefore, the engine must periodically operate in the homogeneous mode to purge even though there is no additional power requirement. Which means that operation in a stratified mode, which is advantageous for fuel economy, is limited by the necessity to purge the canister. Thus, it is advantageous to minimize the purging operation to the lowest acceptable level so that fuel economy can be maximized.
To minimize the purging operation, a measurement of canister saturation could be used so that the canister was purged only when necessary. One approach to monitoring the operating condition of the canister is to use a temperature sensor located in the canister. The temperature sensor senses a temperature rise or fall resulting from adsorption or regeneration, respectively. The temperature can then be monitored to determine the operating condition of the canister. The inlet of the canister is coupled directly to the fuel tank via a valve and the outlet of the canister is leads to the engine, with no hydrocarbon storage between the canister and the engine. Such a system is disclosed in U.S. Pat. No. 5,150,689.
The inventor herein has recognized numerous disadvantages when using the above system to determine when to stop purging operation, i.e., when the canister is emptied. For example, because the canister must be able to store a significant amount of hydrocarbon vapor, there is a relatively large amount of carbon resulting in a large time delay between the actual point of saturation and the resulting measured change in temperature. This large time delay causes less than optimal performance when trying to minimize purging operation.
Another disadvantage inherent in the system proposed in U.S. Pat. No. 5,150,689 is due to the configuration. In particular, vapors received by the canister directly from the fuel tank may or may not be saturated with hydrocarbons. This causes a disturbance in the temperature measurement used for detecting a canister saturation state. For example, the method described in U.S. Pat. No. 5,150,689 may result in a false representation of the state of the canister when there is change in the hydrocarbon content of the vapor entering the canister. In other words, the canister temperature of the canister may stop decreasing because of an increase in the hydrocarbon content of the vapor entering the canister from the fuel tank or because the canister is empty. Thus, the system may erroneously determine that the canister is empty when significant vapors are being generated in the fuel tank. This is a disadvantage because not only is the canister still partially full, but it will fill rapidly and possibly become oversaturated when purge flow is erroneously stopped.
Consequently, erroneous results will be obtained if using a temperature sensor located in a canister in which the primary purpose of the canister is to provide primary storage of hydrocarbons in vapor recovery systems.